1. Field of the Invention
The present invention relates to an optical fiber laser, more particularly to an optical fiber laser in which a photonic bandgap fiber adjusted to produce a photonic bandgap region only in a signal wavelength region is used, and a parasitic oscillation is suppressed.
2. Description of the Related Art
Recent years have seen advances in the high output of fiber lasers. Optical fiber lasers with an output over kW have been developed. Such optical fiber lasers with a high output have come to be utilized in a variety of fields such as in finishing machines, medical equipment, and measuring equipment. Compared with other types of lasers, the optical fiber lasers have an excellent capability to collect light, and hence, are capable of obtaining a very small beam spot with a high power density. Therefore, the optical fiber lasers allow high-precision machining. Machining that uses the optical fiber laser is non-contact machining, and is also capable of machining a hard substance if the substance is capable of absorbing a laser beam. For these and other reasons, the range of applications of the optical fiber lasers is rapidly increasing especially in the field of material machining.
FIG. 14 shows a schematic diagram of a representative high-output optical fiber laser with a system called the MOPA.
In the MOPA system, to the subsequent stage of a master oscillator (hereinafter, sometimes referred to as MO) 100, a power amplifier (hereinafter, sometimes referred to as PA) 200 is connected. With this configuration, a feeble pulsed beam that has been output from the MO 100 is amplified by the PA 200, and a laser beam with a high output is emitted from the PA 200. If a sufficient output is not obtained with a single-stage PA 200, PAs 200 are connected in multiple stages so as to obtain a desired output.
Systems for the MO 100 include: a system in which an output of a CW-oscillating laser light source such as a semiconductor laser is modulated in intensity with a modulator such as an acoustooptical element into pulsed light; and a system in which a fiber ring laser is used such as described, for example, in Patent Document 1.
FIG. 15 shows a schematic block diagram of a representative fiber ring laser.
A fiber ring laser 100 comprises: an excitation light source 101; a WDM coupler 102 for combining an excitation light with a laser beam; a rare-earth-doped optical fiber 103, the rare earth being a gain medium; an isolator 104; an optical switch element 107; and an output coupler 105. The excitation light emitted from the excitation light source 101 enters the rare-earth-doped optical fiber 103 via the WDM coupler 102. The excitation light having entered the rare-earth-doped optical fiber 103 is absorbed into the rare-earth ions doped in the core of the rare-earth-doped optical fiber, to thereby excite the rare-earth ions. The rare-earth ions in the excited state emit spontaneous emission with a specified wavelength. While being amplified, the spontaneous emission propagates through the rare-earth-doped optical fiber 103, and is output as an Amplified Spontaneous Emission (ASE). The WDM coupler 102, the rare-earth-doped optical fiber 103, the isolator 104, the output coupler 105, and the optical switch element 107 are connected in a ring. Therefore, the ASE circulates through these parts, and is again amplified by the rare-earth-doped optical fiber 103. After sufficiently amplified, the ASE laser-oscillates, a part of which is output as a laser beam via the output coupler 105. At this time, if the optical switch element 107 is operated so as to periodically repeat a low loss state and a high loss state, the ASE pulse-oscillates. Thus, a pulse laser output is obtained.
For the PA 200, an amplifier with a configuration as shown in FIG. 16 is used. FIG. 16 shows a configuration of an optical fiber laser with the MOPA system. The laser beam that has been output from the MO 100 enters the PA 200 via an interstage isolator 316, and is output after amplification by the PA 200.
The PA 200 includes: a plurality of excitation light sources 201; an optical coupler 203; a rare-earth-doped optical fiber (rare-earth-doped double-clad fiber) 210; and an isolator 206. As for the excitation light sources 201, the rare-earth-doped optical fiber 210, and the isolator 206, the same ones as those used in the MO 100 may be used. For the optical coupler 203, an optical coupler such as described in Patent Document 2 is used. The optical coupler 203 has: a plurality of excitation ports 202 made of a multi-mode optical fiber; and a signal port 204 made of a single-mode fiber; and further has an exit port 205 that is formed by fusing and drawing the multi-mode optical fiber and the single-mode fiber into an integrated entity. The laser beam emitted from the MO 100 enters at the signal port 204 and is emitted to the core of the rare-earth-doped double-clad fiber 210 via the optical coupler 203. On the other hand, to the optical coupler 203, a plurality of the excitation ports 202 are connected. To each of the excitation ports 202, an excitation light source 201 is connected. Each of the excitation light emitted from each excitation light source 201 enters the first cladding of the rare-earth-doped double-clad fiber 210 via the optical coupler 203. The excitation lights having entered the first cladding are absorbed into the rare-earth ions doped in the core, and a population inversion is formed, to thereby produce stimulated emission. With the stimulated emission, the laser beam propagating through the core is amplified, and is then output via the isolator 206.
In the case of the MO 100 with the MOPA system as shown in FIG. 16, if the rare-earth-doped double-clad fiber 210 of the PA 200 is excited, in a state with signal light not being incident from the MO 100, by the excitation lights emitted from the excitation light sources 201 and reaches a specified population inversion ratio, a parasitic oscillation occurs, and pulses with a very high peak value are generated. The population inversion ratio at which a parasitic oscillation occurs is determined by the reflectances on the entrance side and the exit side of the rare-earth-doped double-clad fiber 210. At some of the population inversion ratios, pulses with a very high peak value by the parasitic oscillation are emitted from the rare-earth-doped double-clad fiber 210 to the optical coupler 203. At this time, there have been problems in that the core of the rare-earth-doped double-clad fiber 210 is damaged by pulses with a very high peak value and that the pulses reach the excitation light source 201 and the MO 100 and thereby damage the excitation light source 201 and the MO 100.
Furthermore, even in the state where pulses are emitted from the MO 100 with a cycle that does not produce a parasitic oscillation in the PA 200, and the optical fiber laser functions normally, there may be a case where reflected light from the outside of the PA 200 output induces a parasitic oscillation while the pulses are input. Normally, the excitation light source 201 of the PA 200 emits the excitation light between the pulses. Therefore, the rare-earth-doped double-clad fiber 210 is in an excited state. Consequently, the ASE is emitted from both sides of the rare-earth-doped double-clad fiber 210. For example, in the case where the optical fiber laser is applied to a material machining, the ASE is emitted onto the material to be machined from the optical fiber laser. At this time, in some of the surface states of the material to be machined, the light reflected off the surface of the material to be machined may again enter the optical fiber laser. Then, an oscillation occurs with this reflected light functioning as seeds, and pulses with a very high peak value are emitted from the rare-earth-doped double-clad fiber 210 to the optical coupler 203. This brings about a problem in that the pulses reach the excitation light sources 201 and the MO 100 and thereby damage the excitation light source 201 and the MO 100.
As described above, in conventional optical fiber lasers, a parasitic oscillation occurs. This has prevented a high population inversion ratio from being achieved, and hence, the energy of the pulse capable of being output from the PA 200 has been limited.
To solve these problems, an isolator is inserted into both sides of the rare-earth-doped optical fiber, to thereby keep the reflectance low and suppress the parasitic oscillation, for example, in the method described in Patent Document 3. Furthermore, a short wavelength pass filter is provided on the exit end of the excitation light source, to thereby prevent the ASE emitted from the rare-earth-doped optical fiber from entering again the rare-earth-doped optical fiber after reflection by the pump laser. That is, in the optical fiber laser described in Patent Document 3, the reflectances on the entrance side and the exit side of the rare-earth-doped optical fiber are suppressed as much as possible, to thereby suppress the parasitic oscillation.
Furthermore, for example, in the method described in Patent Document 4, an optical fiber amplifier is divided into two stages, and an isolator is provided between the stages. At the previous stage of the optical fiber amplifier, a gain thereof is kept low, to thereby suppress a parasitic oscillation. On the other hand, at the subsequent stage of the optical fiber amplifier, a gain thereof is high. However, the ASE emitted from the previous stage of the optical fiber amplifier is always incident therein. Therefore, in the optical fiber laser described in Patent Document 4, an amplification of the ASE is produced, but this does not lead to a parasitic oscillation.
In the optical fiber laser described in Patent Document 5, a fiber Bragg grating (hereinafter, sometimes referred to as FBG) is provided on both ends of a rare-earth-doped double-clad fiber, to thereby construct a resonator. Furthermore, to one of the FBGs, there is connected a multi-mode fiber. An excitation light from an excitation light source enters the rare-earth-doped double-clad fiber via the multi-mode fiber. In the optical fiber laser, a core diameter of the multi-mode fiber is larger than that of the rare-earth-doped double-clad fiber. Therefore, the ASE with unnecessary wavelengths that has entered the multi-mode fiber without being reflected by the FBG has a low percentage of reuniting with the core of the rare-earth-doped double-clad fiber through reflection. Therefore, a parasitic oscillation is suppressed. Even if a parasitic oscillation occurs, the generated pulses enter the multi-mode fiber at first. Therefore, even the ASE is collected on the excitation light source via a lens, its spot diameter becomes large. Consequently, the excitation light source is unlikely to be damaged.
However, in the method described in Patent Document 3, the suppression of the reflectance is, in actuality, approximately 0.001% at best. Therefore, in a comparatively high-output optical fiber laser that emits tens of watts or more laser beam, there is a possibility that a parasitic oscillation will occur with this slight reflection or Rayleigh scattering in the fiber functioning as seeds, no matter much the reflection is suppressed. Furthermore, as for reflected light from the outside that is produced after the laser emission (reflected light on the surface of the material to be machined), the intensity of the reflected light is attenuated by the isolator. However, it is impossible to completely suppress the reflected light. This leads to a possibility of inducing a parasitic oscillation with the slightly remaining reflected light functioning as seeds.
In the method described in Patent Document 4, a high-gain amplifier is provided on the exit side. Therefore, there is a possibility of inducing a parasitic oscillation because reflected light from the outside enters the high-gain amplifier at the beginning. To address this, the use of low-gain amplifiers in multiple stages instead of a high-gain amplifier can be conceived. However, in this case, the higher the output is, the greater the number of the stages is. This results in a complex configuration, and hence, lowers efficiency.
In the method described in Patent Document 5, light with wavelengths not reflected by the FBG has a low percentage of reuniting with the core of the rare-earth-doped double-clad fiber after reflection off the end face of the multi-mode fiber. However, the light does not unite with the core at all. Therefore, the higher the gain of an optical fiber amplifier becomes, the higher the percentage of reuniting with the core is. This leads to a possibility of a parasitic oscillation. Furthermore, as for reflected light from the outside, light with the same wavelength as the reflected wavelength of the FBG is reflected by the FBG. However, light outside the wavelength passes through the FBG and enters the rare-earth-doped double-clad fiber. Therefore, there is a possibility that a parasitic oscillation is induced by the light which has entered the rare-earth-doped double-clad fiber.    Patent Document 1: Japanese Patent No. 2977053    Patent Document 2: U.S. Pat. No. 5,864,644    Patent Document 3: Japanese Patent Publication, First Publication No. H05-136498    Patent Document 4: Japanese Patent No. 2653936    Patent Document 5: Japanese Patent Publication, First Publication No. H10-56227